Our rsync package was shipped with bundled zlib to provide compatibility
with the old-style --compress option up to version 3.1.0. Version 3.1.1 was
released on 2014-06-22 and is shipped by all major distributions now.
So we decided to finally drop the bundled library and ship a package with
system zlib. This also fixes security issues, actual ones and in future. Go
and blame those running old versions if you encounter errors with rsync
3.1.3-3.